A magnet of a ferrite (MO.nFe.sub.2 O.sub.3, wherein M represents Sr or Ba, and n is from 5.0 to 6.2) is generally produced by a process which comprises mixing iron oxide obtained from magnetite or mill scale with an oxide or a carbonate of Sr or Ba, calcining the mixture, pulverizing the calcined material, molding the pulverized material into a particular shape, and sintering the molded product.
The calcining step in further detail is carried out by allowing to react iron oxide with an oxide or a carbonate of Sr or Ba in an inclined rotary kiln to obtain a Sr-ferrite or a Ba-ferrite. In general, magnetite, mill scale, or iron oxide derived from iron chloride or iron sulfide typically has a composition as shown in Table 1.
TABLE 1 __________________________________________________________________________ wt. % T.Fe FeO Fe.sub.2 O.sub.3 Fe.sub.3 O.sub.4 SiO.sub.2 Al.sub.2 O.sub.3 MgO CaO __________________________________________________________________________ magnetite 69.1 -- 1.5 93.7 1.5 0.30 0.60 0.61 mill scale 74.88 67.5 32.6 -- 0.09 0.06 0.003 0.10 Fe-oxide 68.9 -- 98.5 -- 0.05 0.05 -- 0.02 (chloride) Fe-oxide 68.9 -- 98.5 -- 0.30 0.10 -- 0.04 (sulfide) __________________________________________________________________________
The Table reads that both magnetite and mill scale contain, however, a large amount of FeO and Fe.sub.3 O.sub.4 in addition to Fe.sub.2 O.sub.3, and therefore there is required an additional step for completely converting magnetite or mill scale into Fe.sub.2 O.sub.3 by oxidizing those iron oxide sources in an oxygen-containing atmosphere or in air, prior to the adding and mixing of the oxide or carbonate of Ba or Sr. The oxidized magnetite or mill scale requires, however, ascribed to large particle size thereof, a long period of time for milling in the subsequent process, and this was undesirable from the viewpoint of economy and productivity.
The present inventors accordingly examined the cross-sectional structure of the oxidized magnetite and mill scale to find that it consists of a surface layer of Fe.sub.2 O.sub.3 6 .mu.m or less in thickness, and a core composed of a mixed FeO.Fe.sub.3 O.sub.4 phase. We have concluded that it is due to this structure that the oxidized material requires a time-consuming milling process, and, more disadvantageously, that this prevents iron oxide from undergoing complete ferritizing reaction with an oxide or a carbonate of Sr or Ba in the subsequent calcining step.
It was based on those finding that the present inventors have proposed a process described in foregoing Japanese Patent Application Hei-1-81789, which comprises milling a magnetite or a mill scale to a specified particle size or less; completely oxidizing the milled material so as to obtain a product containing Fe.sub.2 O.sub.3 for a specified amount or more; adding to the resulting oxidized product a predetermined oxide or carbonate of Sr or Ba; calcining the mixture., and finally obtaining ferrite magnet through a prescribed process steps.
The iron oxide thus produced in te aforementioned process had, however, yet to be increased in strength as a pellet, since it caused troubles in operation. That is, the pellet was liable to powder at calcining in the inclined rotary kiln and therefore underwent partial melting, which as a consequence adhered on the inner wall of the kiln; or it tended to coalesce into a bulk of about 100 mm in diameter that made the subsequent milling rather difficult.